Electromagnetic test methods are commonly used in evaluation of the integrity of nuclear fuel rod cladding and in the evaluation of the relative performance, such as resistance to corrosion, of cladding materials which are subjected to the reactor core operating environment. In particular, electromagnetic test methods are used to measure the thickness of a layer of corrosion formed on the outer circumferential surface of the fuel rod cladding.
The conventional test for measuring corrosion thickness on a fuel rod uses an eddy current sensor, the probe of which is placed in proximity to the fuel rod cladding, separated only by a layer of corrosion. The probe is coupled to an instrument for displaying the inductive reactance of the probe when in proximity to the fuel rod. The probe of a typical eddy current sensor has a coil of conductive wire which in effect acts as both a transmitter and a receiver of magnetic flux. When the coil is driven with an alternating current, an alternating magnetic flux emanates from the coil. This alternating magnetic flux induces alternating eddy currents in the conductive fuel rod cladding, which is typically made of Zircaloy. These alternating eddy currents in turn produce an alternating magnetic flux which induces an alternating "bucking" current which opposes the alternating drive current through the probe coil. The instrument coupled to the probe is used to measure the resulting decrease in the current flowing through the probe, which decrease is a measure of the inductive reactance of the probe.
The inductive reactance is a function of the distance separating the probe coil from the conductive sample in which the eddy currents are induced. Thus, the inductive reactance of the probe, when in contact with a layer of corrosion at a particular point along the fuel rod cladding, is a measure of corrosion thickness at that point. This separation distance between the cladding and the probe is commonly referred to as the "lift-off".
During normal operation of reactors, it is quite common for the reactor coolant to transport dissolved particles which leach out from the reactor circulation piping and which tend to deposit on the nuclear fuel rods in the form of crud. If the crud deposit contains ferromagnetic material, the presence of this ferromagnetic material will adversely affect the electromagnetic test results without the knowledge of the person conducting such tests. This is because the conventional method for measuring lift-off determines the probe inductive reactance only, not the probe resistance, i.e., the conventional method does not determine the complex impedance of the probe of the eddy current sensor.
At present there is no method for detecting the presence of ferromagnetic material in the crud deposits on the fuel rod cladding at the time when the electromagnetic tests are being conducted. This is because the instrumentation which is employed to perform the electromagnetic test has been designed to determine only the proximity of a metallic conductor to a probe and not the probe complex impedance.